Electrostatically deposited coatings have been used for some time to provide diffuse light emission from incandescent lamps. For example, U.S. Pat. No. 4,081,709, also issued to the assignee of the present invention, describes electrostatic coating of silica on the inner bulb wall of incandescent lamps which is obtained by controlling the particle size and the electrical resistivity of the silica powder. The light diffusion produced in this manner completely hides the lamp filament and other internal lamp structure with little light loss and the aforementioned physical characteristics of the silica powder permit electrostatic deposition to be carried out reliably under varying environment conditions. On the other hand, the proper control of electrical conductivity in said silica powder mixture for uniform powder deposition by electrostatic coating can require addition to the powder mixture of various electrically conductive substances such as H.sub.2 SO.sub.4, SO.sub.2, NaCl, Na.sub.2 SO.sub.4, NaOH or Na.sub.2 O and triethylamine as well as heat treatment of the powder mixture itself or its constituents to reduce moisture content.
It would be understandably beneficial to eliminate such need of additives or heat treatment of the silica powder mixture in order to control the electrical conductivity for electrostatic deposition. If the silica powder mixture for such purpose is less sensitive to moisture content, then its shelf life should also be extended without appreciable agglomeration of the solid particulates. In addition, it would be desirable to achieve such modification in the silica powder mixture without significant cost increase or alteration of the electrostatic deposition process in order to produce a deposited coating having equivalent performance characteristics.